The fabrication of electronic devices such as semiconductor devices involves the step of forming an insulating film called PMDs (premetal dielectrics) between transistor elements and bit lines, between transistor elements and capacitors, between bit lines and capacitors, or between capacitors and metallic wirings, the step of forming an insulating film called IMDs (intermetal dielectrics) between metallic wirings, or the step of embedding of isolation grooves. Siliceous materials are generally used in the formation or embedding of the insulating film. Methods for insulating film formation include CVD methods, sol-gel methods, and siloxane polymer solution coating methods.
An increase in integration density of electronic devices has led to an increasing tendency toward an increase in fineness of grooves on a substrate. For example, when fine grooves having a groove width of not more than 0.2 μm and a ratio of depth to width (hereinafter referred to also as “aspect ratio”) of not less than 2 are embedded with a siliceous material by high-density plasma CVD which can form a good film quality at a relatively low temperature, due to conformal properties inherent in CVD, disadvantageously, voids are likely to be formed within the fine grooves. In order to avoid this unfavorable phenomenon, film formation by high-density plasma CVD and opening by etching should be repeated to form a void-free film. In this method, however, damage to the substrate is unavoidable such as a change in groove shape and material properties by high-density plasma atmosphere. Another method for insulating film formation is such that CVD is carried out under conditions of relatively high temperature and conventional plasma density and, if necessary, baking is carried out at a high temperature. This method, however, cannot meet a demand for a lowering in temperature of the process.
A method for solving such problems inherent in CVD is a sol-gel method or a siloxane polymer solution coating method in which a substrate having fine grooves is coated with a sol-gel liquid or a siloxane polymer solution, and the coating is dried to embed the fine grooves and is then heated for conversion to a siliceous material. In this method, for example, the solution of these materials generally has high viscosity, and, further, a dehydration-dealcohol reaction involved in silica conversion is indispensable. These make it difficult to fully embed the fine grooves with the solution and to embed the fine grooves in such a manner that the density is uniform from the film surface toward the bottom of the fine grooves.
In order to evenly embed the fine grooves, patent document 1 proposes a method in which a perhydropolysilazane solution having a weight average molecular weight of 3000 to 20000 in terms of polystyrene is coated onto a substrate having fine grooves, the coating is dried to embed the fine grooves with the perhydropolysilazane, and the perhydropolysilazane is then heated in a water vapor-containing atmosphere to convert the perhydropolysilazane to a siliceous material which has uniform density from the film surface toward the bottom of the fine grooves.
On the other hand, for the above-described PMDs and IMDs, an increase in speed of the electronic device and an increase in integration density have led to a demand for further lowered permittivity of the material. In particular, a permittivity of not more than 3.5 for PMD applications and a permittivity of not more than 2.5 for IMD applications are desired. The reason for this is that, for phosphoric acid-containing silica (PSG) or boric acid-phosphoric acid-containing silica (BPSG) PMDs formed by CVD, the permittivity is generally not less than 4.2 and in some cases not less than 5. In the perhydropolysilazane-derived siliceous film described in patent document 1, the permittivity is around 4, that is, a permittivity value of inorganic siliceous materials, and a low permittivity desired for PMD or IMD applications cannot be achieved.
Further, as described in nonpatent document 1, in PMDs, the presence of mobile ions, particularly Na+ and K+, even when the amount thereof is very small, results in unstable electrical characteristics and reliability. A conventional method for overcoming this drawback is to incorporate phosphoric acid for electrically trapping mobile ions. In general, however, the addition of phosphoric acid (pentavalent) to an inorganic polysilazane has a fear of producing monosilane (SiH4) which causes firing. For this reason, in the method described in patent document 1, phosphoric acid (pentavalent) cannot be added to the perhydropolysilazane. Even when phosphoric acid can be added safely, since phosphoric acid is highly hygroscopic, it has been generally considered that the addition of phosphoric acid is disadvantageous for lowering the permittivity. Doping of phosphorus after conversion to a siliceous film through the utilization of an ion implantation method as described in patent document 3 and patent document 4 is also considered. In this method, however, doping cannot be uniformly carried out over the whole siliceous film. Further, in this method, an additional process is necessary after film formation. This is disadvantageous in throughput.
Patent document 1: Japanese Patent Laid-Open No. 308090/2000
Patent document 2: Japanese Patent Laid-Open No. 75982/2002
Patent document 3: Japanese Patent Laid-Open No. 319927/2001
Patent document 4: Japanese Patent Laid-Open No. 367980/2002
Nonpatent document 1: Tokuyama, “Erekutoronikusu Gijutsu Zensho (Complete Collection of Electronic Technology) [3] MOS devices” 5th edition, Kogyo Chosakai Publishing Co., Ltd., Aug. 10, 1979, pp 59-64